The invention relates to the use of a multilayer film with polyamide and polyester layers for the production of solar modules.
Solar modules, frequently also referred to as photovoltaic modules, serve for electrical power generation from sunlight and consist of a laminate which comprises a solar cell system as the core layer. This core layer is encased with encapsulation materials which serve as protection against mechanical and weathering-related influences.
In conventional solar modules, the active solar cell is positioned between a front cover and a back cover. The front cover is transparent, generally consists of glass; and is bonded by means of an adhesion promoter layer which often contains an ethylene-vinyl acetate copolymer to the layer comprising the solar cell. The back cover provides electric shielding, serves as protection against weathering influences such as UV light and acts as a moisture barrier.
Film composites composed of fluoropolymer films and polyester may conventionally be employed as a back cover. The fluoropolymer film on the outside provides weathering resistance, while the polyester film is utilized to obtain mechanical stability and electrical insulation properties. A further fluoropolymer film on the inside serves for attachment to the sealing layer of the solar cell system. However, such fluoropolymer films have only low adhesion to the sealing layer which is used as embedding material for the solar cells themselves. In addition, the fluoropolymer film contributes to electrical insulation only to a minor degree, which results in the need to use a comparatively thick polyester film.
WO 2008138022 therefore proposes replacing the two fluoropolymer films in such composites with films of nylon-12 (PA12). In a development thereof, WO 2011066595 proposes that the solar cell-facing thermoplastic layer comprise a light-reflecting filler such as titanium dioxide, while the solar cell-remote thermoplastic layer comprise a second filler such as glass fibres, wollastonite or mica, which brings about a higher thermal conductivity of this layer. Illustrative thermoplastics come from the group of the polyamides, polyesters or blends of polyamide and polyolefin. Explicit mention is made of PA11, PA12 and PA1010, and blends thereof with polyolefins.
However, such thermoplastic combinations do not achieve sufficient adhesion of the layers, and particular mention should be made of the combination of polyesters and polyamides.
EP 1 065 048 A2 describes multilayer composites composed of incompatible polyamides and/or polyesters which are bonded to one another via an adhesion promoter layer. The adhesion promoter layer comprises a graft copolymer which consists of a polyamine moiety and grafted polyamide chains.
This document, however, does not demonstrate that the graft copolymer can also be used in a functional layer rather than in an adhesion promoter layer, and that it is also possible to improve the adhesion between layers comprising considerable amounts of fillers. Furthermore, this document also does not disclose the way in which the adhesion level changes over the course of years in the case of this specific application when a corresponding film is exposed to weathering.